The present invention relates to use of transition materials to attach steel to aluminum. More particularly, the present invention relates to a method for attaching a steel counterweight to an aluminum shaft by projection welding using transition materials. While the invention is subject to a wide range of applications, it is especially suited for use in balancing an aluminum drive shaft for rotation about an axis and will be particularly described in that connection.
Often, it is desirable to employ combinations of materials in order to obtain benefits of the properties of each material. For example, drive shafts for vehicles may be developed of aluminum, which offers substantial weight reduction and reduced rotational inertia over steel drive shafts. However, counterweights must be attachable to the shafts in order to properly balance the shafts. The preferred material for these counterweights is steel, since steel has a significantly greater mass density than aluminum and a much larger piece of aluminum would be required to perform as a counterweight if aluminum were used. Hence, it is desired to combine the light weight, heat conductivity, electrical conductivity, and corrosion resistance properties of aluminum with the greater mass density, high strength, and hardness, of steel. However, joining these two materials presents many problems.
One method of joining aluminum and steel is resistance projection welding. The typical resistance projection welding method of joining aluminum to steel, however, is generally considered infeasible due to formation of undesirable discontinuities or defects along the bond line or weld zone.
Another method of joining aluminum to steel is fusion welding. However, steel and aluminum are of considerably different melting points, making fusion welding difficult. In particular, steel and aluminum form a series of brittle intermetallics. As a result, many problems occur in trying to fusion weld steel to aluminum including discontinuities in the weld line, weak welds between two incompatible materials such as steel and aluminum, breakage due to differential expansion and contraction, diffusion forming brittle intermetallic compounds at the interface between the two materials, high stresses due to residual strains from the welding operation, and galvanic couples that cause corrosion.
It is seen then that there exists a need for a process of attaching steel to aluminum which overcomes the aforementioned problems, including reducing the amount of heat introduced into the aluminum and, thus, the thermal expansion and deformation of the aluminum during attachment to the steel.